The invention described herein was made in the course of a defense contract with the United States Government.
Certain military-type telephone handsets designed for field use must meet very stringent temperature and pressure requirements. For example, one such telephone station set must operate satisfactorily, or be capable of being transported without damage, over a wide range of ambient air pressures ranging from that corresponding to sea level up to a high altitude, for example 50,000 feet. The air pressure inside and outside the instrument casing must be substantially equalized over this entire range of pressures. In addition, it must be capable of being submerged in water to a depth of 6 feet without entry of water and damage to the electronic components and other elements within the instrument casing.
In order to meet these pressurization requirements, some means must be provided to facilitate a substantially unimpeded flow of air through an aperture communicating from the outside to the inside of the instrument casing, while still preventing water under pressure from entering the inside of the casing when it is submerged.